Both Astarion's Book of Hungers and Fall of Netheril are clearly tangentially related to things that were touched on in BG3, and WotC seem keen to milk the game's popularity for all it's worth, so with that in mind, a few possibilities:
Gale's Guide to Working the Weave: New spells, and player-facing mechanics for making your own spells and magic items.
Karlach's Blood War Field Manual: An overview of the Blood War, the factions and main battlegrounds involved, etc. A few ways to work it into a campagin. Possibly new fiend statblocks.
Lae'zel's Atlas of Realmspace: An atlas/gazetteer giving an overview of the celestial bodies beyond Toril.
Shadowheart's Apocrypha: An overview of the various deities of the realms, their Chosen and schemes (i.e. plothooks). Statblocks for some of the more prominent Chosen.
Wyll's Primer on Pacts: A guide for GMs on creating pacts for the various Warlock patrons and how to work them into a campaign, as well as minor deals and bargains to tempt non-warlocks.
Minthara's Underdark Digest: An overview of the Underdark, its domains and ecology.